In the ideal/theoretical case, the time would be Time(H) = Capacity(Ah)/Current(A).
If the capacity is given in amp-hours and current in amps, time will be in hours (charging or discharging).
Let’s see an actual case, 10 Ah battery delivering 1A, would last 10 hours. Or if delivering 10A, it would last for only 1 hour, or if delivering 5A, it would last only for 2 hours.
In other words, you can have “any time” as long as when you multiply it by the current, you get 10Ah (the battery capacity).
It is that simple.
So for a 18650 2500mAh(2.5Ah) battery with a device that draws 500mA(0.5A) you have:
Please take note that most batteries, especially those with circuits, will not work down to 0 Volts as a power supply (if it goes to zero, it will have shorter battery life, or even become dead battery if not charged in time), that’s to say,your circuit will stop working at a set voltage before the battery is fully drained.
We, therefore, will need to times 0.8-0.9 for the calculation:
that’s 2.5Ah/0.5A*0.9=4.5 Hours
What if you know Watts only, you will notice that every device use watt to determine it’s main specifications.
A 5 Watt bulb, 20W Laptop, 100W Motor, 200W Solar Street Light, you just name it.
With 90% Power efficiency for Li-ion/LiPo batteries. Then
Discharging Time=Battery Capacity * Battery Volt*0.9 / Device Watt
5Ah*3.7V*0.9/10W = 1.66 hours
Let’s explain with more examples:
for a 1800mAH 3.7v 18650 battery to power a 3.7V 10W digital device, how to calculate the running time?
for 3.7V 10W device，working current would be 10÷3.7 = 2.7027A = 2702.7 mA
In theory that’s: 1800mAh ÷ 2702.7 mA = 0.666 h = 40 min
In reality that’s: 1800mAh ÷ 2702.7 mA*0.9 = 0.599h = 36 min
Notes: 1A=1000mA mA is current, mAh is Capacity
Or you can use 3.7V*1.8Ah(1800mAh)*0.9/10W=0.599h=36min
Another example: 12V 60Ah battery pack to power 220V 100W light
Working time: 12V*60Ah*0.9/100W=6.48 H
How can calculate Battery capacity given a specific size limitation?
the battery usually comes with a pouch or cylindrical form.
in cylindrical form there is capacity listed for each size:
like for lithium 18650 battery: 2000mAh, 2600mAh,3000mAh, 3350mAh,4050mAh
for lifepo4 26650 battery:2500mAh, 3000mAh,3500mAh
if thickness allows, cylindrical battery cells will always be your first consideration on the table. They are on the shelf and therefore has short lead time and small MOQ.
for thickness less then 18mm, you will need to consider pouch lipo cells, and actually, it just comes with so many types of design, so it’s better to get a general idea of what capacity can you get for a given size.
The size of Lithium polymer battery has a great impact on its capacity, that’s the thickness, width, and length of a battery. The material and Production technology will have an effect on the lipo cell capacity.
The actual capacity will be very complex, but luckily we have a simple and fast calculate formula.
K= mah/mm³, which is a parameter that ranges from 0.07-0.12. for a general calculation, we will set it to 0.1
take 103450 battery cell for example (T=10mm, W=34mm, L=50mm)
that’s 10*34*50*0.1=1700, in reality, it’s about 1800mAh or more.
what about 603450(also called 063450 603450LP)?
that’s 6*34*50*0.1=1020 in actual it will be about 1050mAh
Online Size-Capacity Calculator
capacity less than 500-2000mAh
capacity from 2000mah to 10000mAh
1 the calculator does not apply to capacity less than 500mAh or 2mm less in thickness cells
2 If the capacity is not within the stated capacity range, try the other calculator
3 the capacity if for your reference, given the size, thickness, width, material, voltage, the capacity will still vary greatly from each other.
How to convert Watts to Amps or Amps to Watts or Volts to Watts?
You cannot convert watts to amps, HOWEVER, if you have at least two of the following three: amps, volts or watts then the missing one can be calculated.
The Following Equations can be used to convert between amps, volts, and watts.